The optic nerve is a white matter tract of the central nervous system that transmits visual information from the eye to the brain. Theoptic nerveis composed of retinal ganglion cellaxonsand glial cells. Each optic nervecontains 1.1 to1.3 millionaxonsderived from the retinal ganglion cells. The axons in the optic nerve originate in the retina, acquire a myelin sheath as they leave the lamina cribrosa and finally terminate by synapsing in the lateral geniculate bodies or in the midbrain. The optic nerve is 5 to 6 cm long and can be divided into four parts:

The optic nerve consists of Retinal ganglion cell (RGC) axons + Supportive glial tissue+ Vascular tissue and is surrounded by three layers of meningeal tissue (pia, arachnoid, and dura).

Intraocular Part of the Optic Nerve

The intraocular portion of the optic nerve is the shortest portion of the nerve and is approximately 1 mm long. The intraocular portion further can be divided into prelaminar and laminar segments.

The term “optic disc” usually refers to the ophthalmoscopic view of the nerve head, while the term “papilla” is used to describe the appearance of optic nerve head seen in microscopic sections. The optic disc is oval-shaped and measures approximately 1.5 mm horizontally × 1.75 mm vertically in size. The optic disc has a pale red or yellowish red color. The relative pallor of the disc is observed due to the reflection of light from the myelin sheaths of the optic nerve bundles behind LC as well as from the connective tissue & glial tissues which occupy and lie in front of the sclerochoroidal aperture. And the light reddish tint is due to the presence of a capillary plexus derived from the vessels of the circle of Zinn and central retinal artery.

There is a short funnel-shaped depression in the center of the optic disc, from which the retinal vessels appear to emerge. This depression is known as the physiological cup of optic disc. In about 15% individuals, the optic disc does not show physiological cupping. About 15% of cases of the optic disc do not show physiological cupping.

Fibers are continuous with those of sclera and choroid bridge the sclerochoroidal aperture forming a sieve-like structure which is known as lamina cribrosa. The nonmyelinated axons of the retinal ganglion cells converge and make 90 degrees turn sharply at the optic disc and exit as optic nerve through lamina cribrosa. It must be noted that the complete absence of rod or cones in this area (optic nerve head) makes it insensitive to light and thus it is called the blind spot. It has been estimated that the optic nerve consists of approximately 1 million to 2.22 million such nerve fibers. The nerve fibers are arranged in bundles (800–1,200 fibers are arranged in a single bundle). There are two types of axons in the optic nerve:

1. Majority (approximately 90%) of the axons are less than 1 µm in diameter. These axons originate from the midget ganglion cells, associated with cones.
2. Larger axons are few in number and measure 2 µm to 10 µm in diameter. These axons originate from ganglion cells, associated with rods.

The axons in the optic nerve are arranged in a distinct pattern:

• The peripheral retinal nerve fibers or axons are located in the peripheral portion of the optic nerve,
• The central area of the nerve contains fibers from the posterior or central part of the retina.
• Macular fibers form the papillomacular bundle and enter the disc temporally; they remain temporal for a short distance behind the eye, but as they proceed further posteriorly these fibers become diffusely distributed.
• Nerve fibers arising in the nasal half of the retina cross in the chiasm; axons arising in the temporal half are uncrossed.

Intraorbital Part of the Optic Nerve

Immediately after crossing lamina cribrosa, the optic nerve fibers become myelinated. These myelin sheaths or coverings are secreted by oligodendrocytes. As a result of myelinations, the diameter of these fibers increases to 3 mm (from 1.5 mm of the intraocular part).

Lamina cribrosa acts as a barrier for the oligodendrocytes and thus normally myelinations are not seen in the retina.

Myelin sheaths in the optic nerve are secreted by oligodendrocytes because there are no Schwann cells in the central nervous system.

The length of the intraorbital part of the optic nerve is 25-30 mm. This part of the nerve is comparatively longer than its normal distance from the eyeball to optic canal because of the sinuous or curvy course of the nerve in this part. Here the optic nerve is enclosed by the coverings of the meninges namely outer dura mater, middle arachnoid mater and innermost pia mater. These meningeal coverings extend up to eyeball and blend with sclera and periorbita. The subarachnoid space, between arachnoid and pia mater, around the optic nerve is continuous with intracranial subarachnoid space and contains cerebrospinal fluid (CSF). Among these three coverings, only pia mater continue with the intracranial part of the optic nerve. The orbital fat present in the intraorbital part provides support and act as a cushion for the optic nerve

Considering the meningeal coverings and their relation to the optic nerve, now the following pathologies are easily understood:

Raised intra cranial pressure can cause bulging in of the optic disc, clinically known as papilledema. Absence of Müller cells in optic disc make it susceptible to swelling whereas the surrounding retina largely remains normal.

Inflammation of the eye (for example panophthalmitis) can spread to central nervous system and similarly inflammations of the meninges can involve optic nerve

Intracanalicular Part of the Optic Nerve:

The optic canal is situated in lesser wing of the sphenoid bone. The length of this bony canal is 5 mm. Along with the optic nerve, the ophthalmic artery, postganglionic sympathetic nerve pass through the canal. As mentioned earlier, intracanalicular part of the optic nerve carries three coverings of meninges and dural sheath blends with periorbita lining the canal. This blending fixes the optic nerve. The subdural space (potential space between the dura mater and the arachnoid), does not communicate with subdural space of the brain. But the subarachnoid space (potential space between the arachnoid and the pia mater), is continuous with the corresponding intracranial space and transmits cerebrospinal fluid. Thus, it can provide a potential pathway for the spread of blood, infectious agents, and tumor cells between the eye and the brain

Intracranial Part of the Optic Nerve:

After leaving the bony canal, the optic nerve passes posteriorly and upwards to reach the optic chiasma, which is situated on the floor of the third ventricle.

Optic chiasma is a flattened bundle of nerve fibers, roughly rectangular in shape with a dimension of 12 mm x 8 mm x 2 mm. Fibers from nasal hemiretinas of both eyes decussate here to the contralateral side while the fibers from the temporal hemiretinas remain ipsilateral as continue as an optic tract.

Glial tissue of optic nerve:

The supportive tissue system of the central nervous system (CNS), known as neuroglia or glial tissue, is similar for the optic nerve also. These include astrocytes, oligodendrocytes, and microglia.

Astrocytes: Astrocytes of the optic nerve can be compared with Mullers cells in the retina. They protect the axons structurally and maintains a stable biochemical environment,

Oligodendrocytes: Oligodendrocytes are unique cells found only in optic nerve posterior to the Lamina cribrosa, (normally absent in retina and optic nerve head) which produces myelin sheaths that covers the optic nerve axons.

Microglia: Microglias are the phagocytes of the central nervous system. These cells are found within bundles of axons.